1. Field of the Invention
The invention relates generally to an improved braking system for tractor-trailer combinations, and more specifically to an improved braking system employing a fast-acting booster valve assembly for substantially reducing the fluid pressure delivery time to the brakes of a semi-trailer or trailer.
2. Description of the Prior Art
In a conventional tractor-trailer brake system, it is common practice to control the operation of the trailer brake actuators in accordance with pressure impulses passing through a service line incident to the operation of a manually-operated brake pedal located on the tractor. Due to the fact that there is a finite time lag between the time at which the pressure impulses reach the tractor brake actuators and the time at which the pressure impulses reach the trailer brake actuators, the tractor brakes often engage ahead of the trailer brakes and may result in trailer overrun or "jackknifing".
It has become common practice to provide the tractor with a hand valve for controlling the actuation of the trailer brakes so that the operator can snub the trailer brakes, when desired, or initiate the operation of the trailer brakes prior to the time at which he engages the foot pedal to insure that the trailer brakes are engaged ahead of the tractor brakes. The disadvantage of such hand valves lies in the fact that the overrun problem is most evident in emergency type situations in which the operator would have insufficient time to think of applying the hand valve ahead of the foot pedal.
The prior art recognizes this problem and attempts have been made to compensate for the lag time resulting from the length of the service line between the manually operated control valve in the tractor and the trailer brake actuators. Many of the purposed solutions employ an electrical means responsive to the initial engagement of the foot pedal to transmit an electrical signal prior to the time that the pedal has been engaged sufficiently to actuate the tractor brakes. The electrical signal can be used to operate a solenoid or the like which controls the emergency relay valve to supply fluid pressure to the trailer brake actuators without waiting for the pressure impulses in the service line to reach the emergency relay valve. Such systems suffer from several disadvantages. The tractor itself must be modified to generate the electrical control signals, and electrically responsive valves must be provided on the trailer to control the emergency relay valve. Additionally, both tractors and trailers must be equipped with electrical wires for transmitting the signals from the tractor to the electrically operated valve. The results in a greatly increased initial expense, in increased maintenance costs, and in an additional source of failures within the brake systems. As a result, such electrically-controlled valves have met with little success in the trucking industry.
U.S. Pat. No. 3,001,831, which issued to William Stelzer on Sept. 26, 1961 for a "Tractor-Trailer Brake System", teaches a non-electrical auxiliary valve interposed in the service line and mounted on the trailer just ahead of the emergency relay valve. This auxiliary valve lessens the delivery time of air to the trailer brakes soley because of amplification within the valve. The auxiliary valve is, in commercial practice, mounted on the rear of the trailer just forward of the emergency relay valve. There is still a finite time lag as the pressure impulses travel from the tractor control valve along the service line to the auxiliary valve. Additionally, the valve is relatively slow acting and does not have its own closely-coupled air reservoir. Therefore, while the auxiliary valve does operate to lessen the air delivery time to the trailer brakes, it does not substantially lessen the air delivery time so as to insure that the trailer brakes engage not later than the time which the tractor brakes engage. Trailer overrun and "jackknifing" can still occur; hence a further lessening of the air delivery time is required. Furthermore, complex adjustments must be made to the Stelzer auxiliary valve to prevent trailer "hopping" in deadheading applications since hopping leads to tire wear and possible unsafe conditions. Furthermore, the prior art does not teach braking systems which are especially suited to todays complex semi-trailer and tractor combinations and trains. The present invention provides a fast-acting independent booster-valve and directly coupled air source combination which is located at the start of the air delivery system to insure a faster application of the trailer brakes. The system of the present invention does not require adjustments for dead-heading operations; insures greater compatibility between diverse tractor and trailers; and is less expensive due to the truck-mounting capability which enables a small fleet of tractors to be modified so as to handle a much larger fleet of trailers which may remain unchanged.